[,1] [,2]
[1,]    1    2
[2,]    3    4
[3,]    5    6

[,1] [,2] [,3] [,4] [,5] [,6]
[1,]    1    1    1    2    2    2
[2,]    3    3    3    4    4    4
[3,]    5    5    5    6    6    6

mat[,rep(seq_len(ncol(mat)), each=3)]
#      [,1] [,2] [,3] [,4] [,5] [,6]
# [1,]    1    1    1    2    2    2
# [2,]    3    3    3    4    4    4
# [3,]    5    5    5    6    6    6

(mat <- matrix(1:6, nrow=3, byrow = TRUE))
#      [,1] [,2]
# [1,]    1    2
# [2,]    3    4
# [3,]    5    6

k  =3
matrix(rep(mat, each = k), ncol = ncol(mat) * k, byrow = TRUE)

#     [,1] [,2] [,3] [,4] [,5] [,6]
#[1,]    1    1    1    3    3    3
#[2,]    5    5    5    2    2    2
#[3,]    4    4    4    6    6    6